Spinal rods for immobilizing vertebral bones of the spinal column are typically anchored to the vertebrae via anchor members such as bone screws or hooks. The rods are connected to the anchor members by generally yoke-shaped coupling members that can be integral with the anchor member. Alternatively, the coupling members may be a separate component from the anchor member to form the coupling device for use in polyaxial pedicle screw systems for spinal rod fixation. Generally, these prior systems employ a compression or lock member that is advanced toward the spinal rod for securing it relative to the anchor member, and in polyaxial systems for securing the anchor member relative to the coupling member.
For this purpose, the compression member and coupling device typically have a threading engagement therebetween such that the compression member is threaded down into its locked position in or about the yoke-shaped coupler. In this regard, wedge cam surfaces provided on radial flanges of the compression member and in radial recesses in the upstanding walls of the coupling yoke member have also been employed to advance the compression member for pushing the spinal rod down into fixed position relative to the screw anchor member, see U.S. Pat. No. 6,565,565, to Yuan, et al. The problem with the threaded or cam wedge systems of spinal rod locking is that to allow the compression member to advance relative to the coupler, the size or profile of the coupler as well as the compression member may be increased. In other words, to have threads or cam surfaces formed on the coupler requires that the walls be provided with a sufficient axial extent for the advancement of the threaded or cammed compression member therealong.
In polyaxial spinal fixation systems, the use of inserts between the head of the anchor member and the spinal rod has been proposed, see U.S. Pat. No. 5,733,286 to Errico, et al. The large hemispherical insert of Errico, et al. is engaged on a concave recess formed in a screw head received in the coupler allowing the coupler to adjust relative to the polyaxial pedicle screw for receipt of the spinal rod in its desired position. However, the entire Errico, et al. system has an undesirably large profile as it employs a threaded set screw for clamping on the spinal rod, and the hemispherical insert extends well beyond the top of the screw head into the coupler channel through which the spinal rod is received.
With prior threaded systems, it is often difficult for the surgeon to reposition or adjust the spinal rod relative to the fixation system during the surgical procedure after the compression member is inserted into the cap. Once the compression member is initially threaded into the coupling member, any space between the spinal rod and the compression member for final positioning of the rod prior to locking is randomly achieved. For example, the surgeon can randomly thread the compression member a few turns to provide a re-positioning gap between the compression member and rod or completely thread the compression member into a locking position and then back-off the threading to form this re-positioning gap. As a result, the current threaded systems for immobilizing vertebral bones with spinal rods typically require the surgeon to spend more time and guess work for achieving a gap between the compression member and rod for any final positioning of the rod prior to locking. If the re-positioning gap is not large enough, the spinal rod may bind during repositioning, thereby requiring even additional time and adjustment of the compression member to form a larger gap. All this random threading and guesswork by the surgeon requires additional time in the operating room for performing the surgical procedure on the patient.